Professional photographer User:Katy Blackwood raises an excellent point on her user page and in a personal template, that JPG files tend to be oversharpened by the thumbnail engine, yielding artefacts that lower the quality of sharp images (which is damaging to professionals whose career depends on such details). User:Adam Cuerden has raised that issue during his talk at Wikimania 2016 as well, so the issue is quite widely known. It might be opportune to mention this in the guidelines, so that interested authors could know the workaround and other users understand the reason for the unJPEGness of some files and refrain from uploading JPEG versions. Rama (talk) 10:58, 27 June 2016 (UTC)

I'll be honest - I think we need some sharpening, as ImageMagic tends to produce a blurry thumbnail. But how much it should be sharpened.... I think we're probably on the high side, at the least. PNG is too little, JPEG, a little too much for anything that's not made up of distinct lines. We should NOT suggest workarounds, though; we should fix the problem by tweaking defaults and allowing the sharpening factor to be set for individual images (if possible). Adam Cuerden (talk) 16:02, 27 June 2016 (UTC)